1. Field of the Invention
This invention relates to circuit interrupters and, more particularly, to circuit breakers, which provide thermal and/or overcurrent or instantaneous protection for a load, and more particularly, to molded case circuit breakers, which include a trip unit. The invention also relates to trip units for circuit interrupters, such as circuit breakers.
2. Background Information
In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light industrial applications, overcurrent protection is typically provided by a thermal-magnetic trip device. This trip device typically includes a bimetal strip that is heated and bends in response to a persistent overload condition. The bimetal, in turn, unlatches a spring powered operating mechanism that opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
For short circuit protection, an armature, which is attracted by sizable magnetic forces generated in a magnetic core by a short circuit, unlatches, or trips, a circuit breaker operating mechanism. As an example, the magnetic type actuation occurs when the hot line conductor becomes directly connected with ground or neutral, thereby bypassing the load.
Many circuit breakers employ integral trip units to automatically open the breaker, thereby de-energizing the associated circuit whenever an overcurrent or overload condition exists. Typically, such trip units are electro-magnetic-based. The trip units provide independent protection for each circuit breaker phase through a common trip mechanism, in order that all breaker poles are opened should one pole or phase encounter an overload or overcurrent.
In a conventional thermal-magnetic trip unit, a resistive portion of a phase conductor is used to directly heat the corresponding bimetal. The heated bimetal of the trip unit deflects as the result of i2R heating, wherein i is the circuit breaker's load current and R is the value of resistance of the bimetal heater. The bimetal overcurrent protection is nearly ideal, since the purpose of the trip unit is to protect wire insulation on the protected power circuit conductor from an over-temperature condition caused by the i2R heating of the wire. The bimetal temperature determines its deflection. In turn, at a particular bimetal temperature or deflection, the spring powered operating mechanism is released, in order to trip open the circuit breaker. The bimetal temperature tracks the protected wire's insulation temperature. This, therefore, tends to limit the maximum temperature of that insulation.
The mass of the heater for the bimetal is designed to give a thermal heating response time that again matches that of the protected wire. For relatively large currents, the response time is inversely proportional to the square of the current. The energy absorbed by this heater and, thus, its temperature rise, assuming time is too short for cooling to occur, is simply power integrated over time. For a fixed current and resistance, the temperature rise is proportional to i2RT, wherein T is the time duration of the flow of the current i. The time for a fixed heater/bimetal temperature rise (Δt) as a function of i is, therefore, given by Δt/i2=constant. In other words, for a particular value of current, the time for the temperature to rise to a particular value is also constant.
Once the bimetal has been heated to the circuit breaker's trip value, the breaker trips and, thus, the load current through the bimetal heater is interrupted. In turn, the bimetal begins to cool just as the wire of the protected power circuit cools. This “memory” effect is important. If the circuit breaker is re-closed shortly after it has tripped, then it is important that any subsequent trip occur relatively sooner, in order to again limit the wire insulation temperature to a fixed value.
The circuit breaker trip unit also provides overcurrent or instantaneous protection. The phase current is passed through a magnetic circuit consisting of a moveable magnetic element and a series air gap. The overcurrent produces a flux in this path, which passes through the air gap and the moveable magnetic member, thereby producing a magnetic force in an effort to shorten the air gap by moving the moveable member. The design is such that if the moveable member moves, then it also unlatches the spring powered operating mechanism, thereby tripping open the circuit breaker.
The simple and apparently inexpensive thermal-magnetic trip unit has a number of problems. First, it is difficult to calibrate, since actual currents are required. The bimetal must actually bend in order that its travel can be adjusted to allow the operating mechanism to trip open the circuit breaker at the appropriate current level. If the bimetal adjustment is wrong, then the bimetal must be cooled in order that the calibration can again be verified and adjusted if necessary. This process can be repetitive and time-consuming.
A second problem involves the lack of adjustment or compensation for ambient temperature change. A circuit breaker, which is exposed to an outdoor environment, such as direct sun light, tends to trip at a lower value of current than it would if it were located in a cooler environment. While it is possible to provide an ambient compensated bimetal through the use of a second bimetal, such designs are relatively expensive and are typically not employed in molded case circuit breaker applications.
Finally, it is desirable to allow a limited adjustment of the thermal or overload trip level. For example, if a power circuit is wired for 600 amperes, then a 500-ampere trip unit best protects the connected load. Hence, it is desirable to be able to lower the trip unit's rated current value by, for example, 50% through a simple external adjustment.
There is room for improvement in circuit interrupters and circuit breaker trip units.